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This microreview aims at presenting a personal perspective on the possible contributions of molecular metal carbonyl clusters (MCCs) to a better understanding and development of metal nanoparticles. High‐nuclearity molecular MCCs are perfectly (atomically) monodisperse, ligand‐stabilised metal nanoparticles with nanometric dimensions. Thus, when the nuclearity of molecular clusters is increased and the dimensions of metal nanoparticles are reduced, these two worlds begin to overlap. Herein, the synthesis of larger MCCs is briefly outlined, together with the description of their structural features and physical properties. The use of molecular MCCs as nanometric or subnanometric building blocks for self‐assembly of functional nanomaterials is also reported. Finally, the employment of MCCs as precursors for metal nanoparticles after decomposition of the molecular precursor is described. The purpose of this microreview is to discuss some representative examples of all these topics, showing that one of the most promising and fascinating aspects of molecular MCCs is their borderline nature between molecular chemistry and nanochemistry.

We report on the synthesis and physical chemical characterization of a class of heteroleptic mononuclear cyclometalated bis(phenylpyridine)iridium(III) complexes with tetrazolate chelate ligands, such as the deprotonated form of 2-(1 H-tetrazol-5-yl)pyridine ( PyTzH), 2-(1 H-tetrazol-5-yl)pyrazine ( PzTzH), and 5-bromo-2-(1 H-tetrazol-5-yl)pyridine ( BrPyTzH). The electrochemical and photophysical investigations of the resulting iridium(III) complexes revealed a rather wide span of redox and emission properties as a consequence of the nature of the ancillary tetrazolate ligand. In particular, within a series of the three neutral species, the emission observed changes from the blue-green of the pyridyltetrazolate complex to the red of that containing the pyrazinyltetrazolate ligand. The bromo-containing species, despite it displaying poor photophysical performances, is a synthetically attractive building block for the construction of polymetallic architectures. Moreover, the investigation of the reactivity toward electrophiles of one of the neutral mononuclear complexes, by methylation of the coordinated tetrazolate ligand, has also allowed further tuning of the electronic properties. In the latter case, the emission color tuning is also associated with a simple method for the conversion of a neutral species, a potentially triplet emitter for organic light-emitting devices, into the corresponding methylated cation, which might be used as a dopant for light-emitting electrochemical cell type devices or as a marker for biological labeling.

The sequence of reaction steps and the role of each reactant, required for the transformation of the Pd( 0] ϩ , 2a, the active Pd()-hydride catalyst for the methoxycarbonylation of ethene to methylpropanoate, have been delineated using a combination of spectroscopic and crystallographic methods. The preparation and characterisation of a variety of related complexes are described including some unusual examples involving bidentate sulfonate complexes and mono-cationic and neutral palladium hydride complexes. X-Ray crystal structures have been determined for [Pd(2-(CH 2 PCy 2 ) 2 C 6 H 4 ], 7, and [Pd(d t bpx)(η 2 -MeSO 3 )] ϩ , 9b.

Diiron μ-aminocarbyne compounds, 1a-e, are prepared in two steps from Fe 2 Cp 2 (CO) 4 , negating the need for difficult purification procedures of intermediate species; they are efficiently isolated by alumina chromatography. Minor amounts of μ-aminocarbyne aryl isocyanide compounds, 2a-c, are obtained as side products. The structures of the cations in 1a,c,e are calculated using DFT; the carbyne carbon is generally predicted to be the thermodynamic site of hydride addition, in agreement with a previous experimental finding concerning 1a. Accordingly, the reaction of 1e with NaBH 4 affords a bridging aminocarbene complex, 4, in 85 % yield. Otherwise, the reaction of 1c with NaBH 4 yields the aminocarbyne-cyclopentadiene derivative 3 (70 %), presumably as a consequence of the [a] Scheme 1. Regioselective additions of nucleophiles to the diiron aminocarbyne complex 1a. Results and DiscussionThe commercial compound [Fe 2 Cp 2 (CO) 4 ] was reacted with the appropriate isocyanide, in a ca. 3:2 molar ratio, in acetonitrile solution. [16] The reactions with alkyl isocyanides were conducted under reflux conditions, whereas the reactions with aryl isocyanides proceeded at room temperature. The resulting mixtures were dried under vacuum and the residues were dissolved in dichloromethane and then treated with methyl triflate, thus affording the μ-aminocarbyne complexes 1a-e (Scheme 2). The difficult isolation of the monoisocyanide intermediates (see the Introduction) was unnecessary. The final products 1a-e were efficiently purified by alumina chromatography and were then isolated as microcrystalline, air-stable compounds in 65-92 % yields. The synthesis of 1c-e was accompanied by the side formation of minor products derived from di-isocyanide species, 2a-c. Compounds 2a-c were recovered by the chromatography in 3-12 % yields, although 2a was formerly reported as being Scheme 2. Synthesis of diiron μ-aminocarbyne complexes.Eur. J. Inorg. Chem. 2018, 960-971 www.eurjic.org

A detailed spectroscopic study has allowed the solution structure and dynamic properties of all the intermediates in the Pd-catalyzed methoxycarbonylation of ethene to be established. [Pd(L−L)H(solv)]+ 1 (L−L = 1,2-(CH2PBut 2)2C6H4; solv = MeOH, 1a; PrnOH, 1b; THF, 1c; EtCN, 1d) is static, and the two inequivalent P atoms do not become equivalent through solvent exchange over all the temperatures studied. 2, contains a strong β-agostic C−H interaction which is remarkably stable and is not displaced even in strongly coordinating solvents such as EtCN. Cα and Cβ of the ethyl group in 2 become equivalent via a stereospecific interchange involving [Pd(L−L)H(η 2-C2H4)]+ without making the two P atoms equivalent; at higher temperatures these two inequivalent P atoms do become equivalent probably via a T-shaped intermediate. For [Pd(L−L)(C(O)Et)(solv)]+, 6, there is no β-agostic C−H interaction and multiple 13C-labeling of the C(O)Et group shows that the inequivalent P atoms become equivalent via movement of the intact C(O)Et group. The crystal structure of the related complex [Pd(L−L)(C(O)Et)Cl] cocrystallized with dibenzylacetone has been determined.

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